This invention relates to optical objectives, and more particularly, to a telephoto objective having a stationary front lens group of positive power and a rear lens group of negative power movable for focusing.
Although most optical objectives are bodily movable for focusing purposes, the focusing can be otherwise achieved by axial movement of either the front or the rear lens group of the optical objective.
In telephoto objectives, if made bodily movable for focusing, the amount of axial movement of the objective throughout the entire focusing range tends to increase, and this, in turn, calls for increases in the weight and bulk of the objective and the driving torque of its focusing mechanism such as those including helicoid structure as well as in the production cost thereof, thereby it being made more difficult to keep the weight and bulk of the complete objective including its mechanical mounting and focusing mechanism within easily manageable proportions.
In the case of a telephoto objective of the type in which the provision for focusing is made at a rear lens group thereof, it is possible to minimize the amount of focusing movement and further to reduce the size and cost of the focusing mechanism therefor, because of the relatively small weight and bulk of the rear lens group, which in turn results in a reduction of the driving torque which would be otherwise necessary for the focusing thereof. This provides an advantage of facilitating high-speed focus adjustment of the objective. Another advantage of this type telephoto objective is that the front lens group which is of a relatively large weight and bulk may be supported in fixedly secured relation with a camera body to minimize the probability of producing a jiggle or oscillation of the image at the focal plane by small accidental motions of the objective and camera assembly which is otherwise encountered, particularly when the objective is telephoto in nature. This type of telephoto objective is, however, susceptible to large variation of aberrations and particularly spherical aberration during focusing, and consequently high grade imagery cannot be preserved throughout. This is particularly true when the objective is constructed as comprising a front lens group which has a positive power and a rear lens group which has a negative power for the purpose of minimizing the overall length of the objective. In the objective of such construction, the front lens group produces a negative spherical aberration, while the rear lens group produces a positive spherical aberration, whereby the best balance between them is usually effected at a focusing position for an object at infinity. As the objective is focused from the infinity distant position to a close distance position, however, the height of incidence of an axial ray upon the rear lens group is decreased causing the rear lens group to contribute a decreased positive spherical aberration to the whole lens system. As a result, the image spherical aberration is remarkably undercorrected.